docs: def CA_TRACE(residue):
      """
      Reduces protein residues to the alpha carbon:
      CA trace only.
      """
  
      cg_residue = deepcopy(residue)
  
      for atom in cg_residue.child_dict.keys():
          if cg_residue[atom].name != 'CA':
              cg_residue.detach_child(cg_residue[atom].name)
      
      return cg_residue